The present invention generally relates to a testing technique and in particular to a testing technique involving alkali halide crystals with anisotropic centers for capability as holographic media and for determining their optimum operating conditions.
Alkali halide crystals with anisotropic centers have shown promise as holographic recording and storage media in U.S. Pat. Nos. 3,580,688, 3,673,578, 3,720,926, 3,727,194, 3,771,150, 3,814,601, and 3,846,764. The advantages of these crystals include an on-line read-write-erase storage capability, minimal fatigue, reasonable writing sensitivity, nondestructive read, and thermal stability. The response of holographic media in wavefront reconstruction is generally characterized by diffraction efficiency which is defined as the ratio of diffracted to incident light intensity. As the diffraction efficiency increases, the reconstructed holograms are brighter.
Diffraction of light by a crystal is produced through general grating array variations of either absorption or refractive index. Phase patterns are clearly preferred since they generally give rise to much larger diffraction efficiencies in wavefront reconstruction.
The characterization of absorption differences is easily determined through standard spectrophotometric procedures, particularly for anisotropic centers which give rise to distinct easily measurable absorption bands in the visible, near infrared, and near ultraviolet spectral regions. However, the contributions of anisotropic centers to the refractive index of a crystal has been extremely difficult to measure because of their relative small contribution to the overall refractive index. Thus, it becomes difficult to accurately predict the phase contribution of anisotropic centers to the diffraction efficiency and therefore to properly prepare the crystal and to select the optimum operating parameters for use as a holographic recording medium. Techniques to determine this contribution such as Faraday rotation are usually cumbersome requiring very low operating temperatures and high magnetic fields and have been useful primarily in special cases involving highly isotropic centers such as F-centers, but not for the class of centers which are anisotropic. This lack of a quick and simple technique to directly measure the contribution to the index of refraction by anisotropic centers and thus their dispersion may very well be the underlying reason for the oversight of the predominant effect these centers have on the crystal diffraction.